Bubbling pondweed
Teaching Notes
Introduction
Note: Cabomba caroliniana is no longer available in the EU, due to new regulations on
the propagation of invasive species. Please see the associated document “Pondweed
species” on the SAPS website for advice on which species of pondweed to use for
this protocol.
Like many other water plants, pondweed has specialised tissue called aerenchyma that allows gases
to diffuse inside the plant. Aerenchyma provide buoyancy and allows oxygen transport to the
submerged roots that may otherwise be in hypoxic (low oxygen) conditions. If you cut through the
aerenchyma, oxygen can escape.
This can be made use of this in the classroom to show how the rate of photosynthesis is affected by
light intensity or by the colour of light reaching the plant.
This document will explain how students can investigate the rate of photosynthesis by monitoring
oxygen generation in pondweed. It includes notes on how to do this by:
1)
2)
3)
4)
Counting pondweed bubbles in white light
Counting pondweed bubbles in different wavelengths (colours) of light
Collecting pondweed bubbles in a pipette
Collecting pondweed bubbles in a microsyringe
Demo video
We have produced a demo video showing how to get the most out of this protocol in the lab, available
at https://www.youtube.com/watch?v=eIEJ0FfB-VI
Safety and biosecurity
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
Take care with use of water near electrical equipment. Ensure hands are dry when using
lamps
Keep all containers of water away from sockets and from areas where spills could drip into
sockets. Boiling tube racks could be placed into trays to catch any spills and prevent water
running onto electrical equipment.
Pondweed should be disposed of in domestic rubbish and not put into ponds or waterways .
Apparatus
Per group to set up a piece of bubbling pondweed
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
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Approx 7cm long piece of pondweed
Boiling tube
Boiling tube rack (type that allows light to illuminate whole tube)
Lamp (see ‘lighting’ in the technical notes)
Meter ruler / measuring tape
Stop clock
Science & Plants for Schools: www.saps.org.uk
Bubbling pondweed - Teaching notes p.1
Resource 190, Version 1.2 (revised 2019)

1% sodium hydrogencarbonate solution (using sodium hydrogen carbonate solution will ensure
that CO2 does not become a rate limiting factor although these plants will bubble well for some
time in water).
To save time, the pondweed could be cut into 5-7cm long sprigs before the lesson, ensuring the
sprigs are returned to a large beaker/trough of water as soon as they have been cut. Cutting the
stems under water will reduce air traps forming in the plant tissues. Each sprig should have only one
cut end and the rest of the sprig should have several healthy ‘leaves’. Students will use these sprigs
upside down (i.e., with the cut end of stem at the top and the ‘leaves’ at the bottom) in their boiling
tube to allow oxygen bubbles to leave the cut tissues.
Basic method
To set up a piece of bubbling pondweed:
1.
2.
3.
4.
Half fill a boiling tube with 1% sodium hydrogen carbonate (or water)
Place a piece of pondweed into the boiling tube (upside down – cut end up)
Gently push the pondweed down, taking care not to snap the stem
Pondweed has rubbery leaves that grip the side of the boiling tube so it
does not need weighing down
5. Place the boiling tube in a rack
6. Position a lamp to shine onto the plant
7. Bubbles of gas should be seen emerging from the cut end after a short time
(one minute is usually long enough)
If bubbles do not appear after 2 minutes, the cut end has probably started to ‘heal’
up, so it will need to be snipped off again to freshen it up
Snipping the cut end
under water to refresh
the sample
Science & Plants for Schools: www.saps.org.uk
Bubbling pondweed - Teaching notes p.2
Resource 190, Version 1.2 (revised 2019)
Investigation 1: counting gas bubbles at different light intensities
Photosynthesis is a chemical reaction. Students should already be aware from
other science lessons that several external factors can affect the rate of a
reaction. These are known as rate limiting factors and most GCSE/14-16 students
need to be able to identify and discuss these factors. The easiest rate limiting
factor to investigate using bubbling pondweed is light intensity.
Method
Students will require the apparatus listed on page 1 and should set up their
pondweed stems in boiling tubes of water or 1% sodium hydrogen carbonate (to ensure that CO2
does not become a rate limiting factor).
Students move their pondweed set distances away from the lamp and count the number of bubbles
that emerge from the cut end of the plant.
They should find that more bubbles are generated in a set time when the pondweed is closer to the
lamp. Small intervals of 2-5cm work best.
Sample data set
Lamp Distance
from plant (cm)
Number of bubbles per minute
Test 1
Test 2
Average
0
2
4
6
8
10
105
95
88
74
66
51
112
91
80
70
56
45
This continuous data can be plotted on a
line graph with a line/curve of best fit
108.5
93.0
84.0
72.0
61.0
48.0
Discussing the data
It frequently occurs that the second reading in each repeat is lower than the first.
Possible reasons that can be discussed

The plant was still adjusting to the lower light intensity during the first reading

The plant was still adjusting to the lower temperature caused by moving away from a filament
lamp

The cut end of the plant may be healing as time progresses

There could be another rate limiting factor as time progresses
Evaluating the investigation
Students who have been well trained in fair testing can be very vocal about the shortcomings of this
method, which can lead to effective ‘How Science Works’ discussions.
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Was it really possible to get the lamp at 0cm?
How could you really make sure the plant had adjusted to the new light intensity each time?
Were there some variables we could not control even though we should have?
Did temperature play a large part in this investigation?
If the lamp had produced no heat, how might this have changed the shape of the graph?
Would energy saving bulbs reduce the heating effect?
Were all the bubbles the same size?
Why is it more sensible to compare the shapes of the graphs in the class rather than to
compare the numbers of bubbles?
Science & Plants for Schools: www.saps.org.uk
Bubbling pondweed - Teaching notes p.3
Resource 190, Version 1.2 (revised 2019)
Investigation 2: counting oxygen bubbles at different wavelengths
of light
Photosynthesis is a chemical reaction. Students should already be aware from other science lessons
that several external factors can affect the rate of a reaction. These are known as rate limiting factors
and most GCSE/14-16 students need to be able to identify and discuss these factors. An interesting
factor to investigate using bubbling pondweed is how wavelength (colour) of light affects the rate of
photosynthesis.
Method
Students will require the apparatus listed on page 1 and:
 Scissors
 Sticky tape
 Acetate film in clear / red / blue / green
 1% sodium hydrogen carbonate (or water)
Students set up their pondweed stems in boiling tubes of 1% sodium hydrogen carbonate.
Students then place their pondweed a set distance (e.g. 5cm) away from the lamp.
Starting with clear acetate, students wrap each of the different coloured acetates in turn around the
boiling tube, and secure with sticky tape.
With the sticky tape facing away from the lamp, they should then record the number of bubbles
produced by the pondweed in a set time (e.g. a minute) in each of the different light conditions.
Students should find that green acetate causes photosynthesis to stop as this light is reflected by
chlorophyll.
Sample data
Filter (acetate)
colour
Clear / none
Red
Blue
Green
Colour of light
getting into the
boiling tube
All colours
Red
Blue
Green
Number of bubbles per minute
Test 1
Test 2
Average
80
42
50
10
84
48
56
8
82
46
54
9
Discussing the data

It may occur that the second reading in each repeat is higher than the first.
Possible reasons:
o The plant was warming up which increased photosynthesis
 It is usually the case that there is still some photosynthesis in green light
Possible reasons:
o The plant has not fully adjusted yet
o Small amounts of red or blue light are getting through the acetate
o The plant contains a photosynthetic pigment that can absorb green light (this is true of
some algae, although these tend to be red algae)
 The difference between photosynthesis in red and blue light can also be discussed
o Is the difference significant? How could this be tested further?
o Can we link this potential difference to the environment the plant lives in?
Science & Plants for Schools: www.saps.org.uk
Bubbling pondweed - Teaching notes p.4
Resource 190, Version 1.2 (revised 2019)
Investigation 3: measuring the volume of oxygen with a pipette
Photosynthesis is a chemical reaction. Students should be aware from previous science lessons that
several external factors can affect the rate of a reaction. These are known as rate limiting factors and
most GCSE/14-16 students need to be able to identify and discuss these factors. The easiest rate
limiting factors to investigate using bubbling pondweed is light intensity.
Two methods are outlined that will allow students to determine the volume of oxygen produced in
different light intensities. Investigation 3 uses disposable plastic pipettes and Investigation 4 uses
microsyringes.
This investigation allows students to measure the volume of gas produced during photosynthesis
more accurately than by counting bubbles. In order to produce quantities of oxygen that can be
measured, plants may have to be left for longer time periods e.g. an hour. Students may therefore
have to set up several sprigs of pondweed at different distances from the lamp
Students will require all of the apparatus listed on page 1 and should set up their
pondweed stems in boiling tubes containing 1% sodium hydrogen carbonate (or
water). They will also require:

Scissors

Blu-tackTM

Small beakers or weighing boats

Disposable plastic pipettes (3cm 3) (2)
Students fill a plastic pipettes with 1% sodium hydrogen carbonate solution so that
there is still air in the bulb of the pipette.
With the tip pointing up so that the solution pours backwards into the bulb, they then
dry the tip end of the pipette using a paper towel, and then seal the end using blutack.
With the tip pointing down students then snip off the bulb of the pipette, catching the
spilled liquid in a small beaker or weighing boat. The snipped pipette can be topped
up with the spilled liquid and then inverted over a cut pondweed stem in a boiling
tube of 1% sodium hydrogen carbonate solution.
If placed near a lamp, oxygen generated by the plant will build up in the pipette
below the blu-tack seal and can be measured using the volumetric markings on the
pipette. Students could set up several of these at different illumination levels to
compare the volumes of oxygen produced.
This could be organised as a class practical where results are collated to produce a
class set.
Suggested discussion points





Plotting a graph of distance from lamp against volume of oxygen generated
Plotting a graph of light intensity (1/(D2)) against volume of oxygen produced
Fairness if collating class data (could control variables be controlled?)
Fairness of the test if different sprigs had to be used for each distance
How to test that the gas is oxygen
Science & Plants for Schools: www.saps.org.uk
Bubbling pondweed - Teaching notes p.5
Resource 190, Version 1.2 (revised 2019)
Tip of pipette sealed with
Blu-tack. The pipette bulb
has been cut off and the
cut end placed over the
cut pondweed stem
Investigation 4: measuring the volume of gas with a microsyringe
Photosynthesis is a chemical reaction. Students should be aware from previous science lessons that
several external factors can affect the rate of a reaction. These are known as rate limiting factors and
most GCSE/14-16 students need to be able to identify and discuss these factors. The easiest rate
limiting factor to investigate using bubbling pondweed is light intensity.
This investigation allows students to measure the volume of gas produced during
photosynthesis more accurately than by counting bubbles. In order to produce
quantities of oxygen that can be measured, plants may have to be left for longer
time periods e.g. an hour. Students may therefore have to set up several sprigs of
pondweed at different distances from the lamp.
Students will require the apparatus listed on page 1 together with:

Blu-tackTM

Microsyringes (2) and tips (1)

Clothes peg (1)

Beaker (1)
Students seal off the end of a dry microsyringe with blu-tack. They then use
another microsyringe fitted with a tip to fill the first microsyringe with 1% sodium
hydrogen carbonate solution.
This is then inverted over a cut pondweed stem in a boiling tube of 1% sodium
hydrogen carbonate solution.
If placed near a lamp, gas generated by the plant will now build up in the
microsyringe below the blu-tack seal and can be measured using the volumetric
markings on the microsyringe.
Students could set up several of these at different illumination levels to compare
the volumes of gas produced. If required, the filled microsyringe can be balanced
over a beaker using a peg while the pondweed is set up if required.
Suggested discussion points





Plotting a graph of distance from lamp against volume of oxygen generated
Plotting a graph of light intensity (1/(D2)) against volume of oxygen produced
Fairness if collating class data (could control variables be controlled?)
Fairness of the test if different sprigs had to be used for each distance
How to test that the gas is oxygen
Science & Plants for Schools: www.saps.org.uk
Bubbling pondweed - Teaching notes p.6
Resource 190, Version 1.2 (revised 2019)
Filled microsyringe
sealed with blu-tack
and placed over the
cut pondweed stem to
collect oxygen.